PROPOSAL SUMMARY The innate immune system responds to perturbations in tissue homeostasis resultant from infection but also dying cells and tissue damage. Cells die by different modes of programmed cell death including apoptosis and necroptosis. Cell death by apoptosis is a normal component of healthy tissue physiology that is balanced by cell division and maintains normal tissue size and function. Cell death by apoptosis is tolerogenic and non- inflammatory, and contrasts with necroptosis, which is pathological and inflammatory. Necroptosis involves the formation of a complex of receptor interacting serine/threonine protein kinases RIPK1, RIPK3 and the necroptosis effector mixed lineage kinase domain-like protein (MLKL). Up until recently, necroptosis was thought to function primarily in the control of infection and mainly as a fail-safe strategy that counters viral blockade of apoptosis. Studies in mouse models, however, have shown that deficiency in components of the apoptosis machinery such as caspase-8 or FADD leads to embryonic lethality driven by necroptosis and dependent on RIPK3 and MLKL. Other studies using conditional deletion of caspase-8 or FADD in the intestinal epithelium revealed that blocking the pathways that mediate homeostatic apoptosis precipitates intestinal inflammation associated with elevated levels of RIPK3 and necroptosis. Indeed, a notable increase in programmed cell death of intestinal epithelial cells (IEC) has been reported in patients with inflammatory bowel disease (IBD), and this damage is associated with heightened inflammation and increased levels of tumor necrosis factor (TNF)-a, an important mediator of cell death. Using a novel mouse model where we can inducibly trigger necroptosis of IEC, we will determine how IEC necroptosis impacts intestinal homeostasis and we will define the nature of the inflammatory response. Our previous work has established that homeostatic apoptosis within the intestinal epithelium is a major driver of immune suppression and tolerance, imprinting intestinal mononuclear phagocytes (MNP) with ?suppression of inflammation? and ?induction of regulatory CD4 T cell? transcriptional signatures. Here we will define how necroptosis impacts MNP responses. We will examine the composition and characteristics of the small intestinal MNP population that responds to necroptotic IEC and decipher its function in various innate immune functions relating to the production of inflammatory mediators, modulation of innate lymphoid cell function as well as the CD4 T helper cell differentiation. The knowledge we gain will define how necroptosis drives inflammatory responses by innate and adaptive populations of cells, serve as a roadmap for the consequences of necroptosis in other tissues, and lay the foundation for the development of novel therapeutics for chronic inflammatory diseases such as IBD.